An optical fiber typically carries multiple spectral channels for transporting signals having different wavelengths. Each of the spectral channels has a fixed center wavelength with a bandwidth size. Each spectral channel is capable of handling and transporting separate and independent information carried in optical signals at different wavelengths. The center wavelengths of adjacent channels are spaced at a predetermined channel spacing (e.g. 50 GHz, 100 GHz or a flexible spacing). The spectral channels may be wavelength division multiplexed to create a composite multi-channel signal travelling through an optical network. At various degrees, locations, or nodes, in the optical network, one or more spectral channels may be dropped from or added to the composite multi-channel optical signal. For example, a reconfigurable optical add-drop multiplexer (ROADM) can be used as an optical multiplexer capable of adding or dropping channels at the wavelength layer.
One technique to add or drop channels of signals is to use Multicast Switch (MCS). FIG. 1 illustrates an optical switching device using MCS and optical amplifier array. The optical amplifier is an Erbium Doped Fibre Amplifier (EDFA). The 1×M (i.e., 1 input port and M output ports) MCS switch can drop any input signals from any direction (E, W, N, or S direction). However, such a device using MCS has various disadvantages. MCS introduces large insertion loss due to the broadcasting internal function. Consequently, to lower the insertion loss, the device has to use wavelength selective switches (WSS) on the route side, as shown in FIG. 1. The WSS switches increase the cost significantly. The device needs to use many WSS and many pairs of MCS and EDFA array for routing/selecting and adding/dropping optical signals. For instance, a simple 4-degree device, as shown in FIG. 1, has 8 1×23 WSS and a couple of MCS and EDFA array combinations. As a result, the device is costly and has a large size.
Another technique to add or drop channels of signals is to use reflective microelectromechanical systems (MEMS) mirror arrays. Two groups of reflective MEMS mirror arrays are used to switch optical signals by adjusting the mirror angles. Because large switching mirror angles are needed to maintain reasonable wide channel bandwidth, such a system is costly and bulky. The system is not reliable since a failure of one of the MEMS component will cause the entire system to fail.
Yet another technique to add or drop channels of signals is to use reflective LCoS chips. LCoS chips' phase modulation functions are used to control the routing of the optical signals with respective wavelengths. The phase-modulated optical signals are freely deflected by multiple small pixels. However, multiple ports usually share the same LCoS chip, which leads to serious crosstalk between these ports. The insertion loss for each port is high due to the low diffraction efficiency out of limited number of pixels to modulate the LC pixels.